A bulk acoustic wave (BAW) resonator typically consists of a thin layer of piezoelectric material sandwiched between two thin electrodes. When an alternating electrical voltage is applied between the two electrodes, the consequent electric field between the electrodes interacts with the piezoelectric material to generate acoustic waves within the piezoelectric material. The resonant frequency of a BAW device depends on multiple factors, whereas the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. The fundamental resonance occurs when the wavelength of the excited mechanical wave is about twice the thickness of the piezoelectric layer. As the thickness of the piezoelectric layer is reduced, the resonant frequency is increased. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonant frequency to 0.5-20 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or film bulk acoustic resonators (FBARs). With resonators as circuit building blocks, networks of resonators can be designed to form ladder, lattice, or other similar circuit forms to implement various filter characteristics. The ladder filter has the parallel resonance of the shunt resonators approximately aligned with the series resonant frequency of the series resonators to form a pass band. The out of band rejection of the ladder filter is controlled by the capacitive voltage divide nature of the ladder circuit when the resonators are operating as simple capacitors. The lattice filter is a cross-over network with a balanced input port and a balanced output port, and is suitable to address fully balanced filtering.
The resonators in both ladder and lattice filters are electrically connected to achieve predetermined signal filtering. Actually resonators may be acoustically coupled to yield more or less classical filter response. One of the primary thickness-mode-coupled resonators is the stacked crystal filter (SCF). An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. An SCF exhibits a narrower bandwidth than that obtained in a ladder or lattice filter. The limited bandwidth of the SCF can be overcome by reducing the acoustic coupling between the vertically disposed transducers in such a way that they begin to act as independent resonators rather than a single resonator. The resulting configuration is called coupled resonator filter (CRF), which encloses a pair of BAW resonators vertically stacked with an acoustic decoupler disposed between the resonators. The acoustic decoupler can take a variety of forms with the goal to partially isolate one resonator from the other. Quarter-wavelength-layer alternating sequences of high and low acoustic impedance materials provide one option and may be of the same material types as used in a reflector stack. A single layer of low acoustic impedance material to form the decoupler is an alternative approach, which departs from the use of the acoustic reflector stack. In a CRF, the amount of acoustic coupling between resonators is used to control filter bandwidth. If there is too great a degree of isolation between resonators, insertion loss is high and the bandwidth is too narrow and the filter cannot meet the bandwidth requirement. If the coupling is too strong, a filter with a wide bandwidth and pronounced mid-band sag is produced. A CRF exhibits slow roll-off of the filtering function outside the filter pass-band, which makes it very difficult to meet the stringent near-band rejection specifications in the applications such as PCS and UMTS-8 duplexers with very small separation between transmit and receive bands. There exists a decoupling layer capacitance in a CRF which arises between the two electrodes enclosing the acoustic decoupler, which could be beneficial to achieve a targeted filter response if properly configured. On the other hand, the existence of the decoupling layer capacitance in a CRF performing an unbalanced to balanced mode transformation greatly compromises the imbalance performance of the filter because it creates asymmetric port-to-ground or feedback capacitance at the balanced output port.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.